1. Field of Invention
The present invention relates to a process for generating milk froth from milk power and hot water using a milk froth generating device.
2. Description of Related Art
For frothing up milk to prepare a cappuccino, the espresso machines are in most cases provided with steam nozzles. For the generation of milk froth, the outlet of the steam nozzle is guided manually over the surface of milk prepared in a vessel so that when releasing the steam, a three-phase zone comprising steam-air-milk is formed generating the milk froth. In most cases, this process can optimally be mastered by professional users only.
For this reason, so-called frothing aids have been developed wherein the steam outlet is simply submerged under the surface of the milk, and by means of a Venturi tube provided in the frothing aid via an air supply duct remaining above the milk surface, air is drawn in the steam outlet by the under pressure generated by the flowing steam so that the three-phase zone is formed under the milk surface and frothing of milk is thus also made possible for users having no practice.
It is a disadvantage of the two methods that both the simple steam nozzles and the frothing aids will be heated in view of the passing hot steam that much that milk remains may stick to the parts of the nozzle in contact with the milk and may even get burned. In case of simple steam nozzles, this contact occurs because when the milk froth is being generated, the three-phase zone is increasingly below the frothing foam and the milk froth will, therefore, stick to the nozzle. In the case of the frothing aids, sticking together is caused by the system considering that the frothing aids are submerged in the milk. In both cases, the soiled parts of the device have to be thoroughly cleaned after use. While nozzles made of poorly heat conducting material might make improvements possible by a smaller tendency to sticking, they do not, however, solve the soiling and cleaning problem.
One cleaning problem is also observed in case of prior art milk frothing systems where by means of the passing steam milk is drawn via a hose duct from a container into a frothing chamber in which it is frothed up and continues to flow directly into the drinking cup. In these systems, hose supply ducts and frothing system have to be thoroughly rinsed with water in order to avoid soiling by milk remains.
Milk froth generation from fresh milk causes a further problem. On one hand, the fat content of the milk determines the quality of the milk froth being made as to pore size and stability, and on the other, the milk should be relatively cold. The keeping of defined fat content limits and temperature limits of the milk is therefore of significance for the frothing result. A further disadvantage of milk froth generation based on cold fresh milk results from the fact that while the hot steam warms during the course of the generation of the milk froth the remaining milk in the frothing vessel, the remaining milk which in most cases, for instance in case of a cappuccino, is added to the coffee in addition to the milk froth displayed on the coffee will not reach the coffee temperature and will, therefore, cool down the coffee, which is highly undesired.
The state of the art includes also a milk froth generating device of a house hold coffee machine by means of which milk frothing should be simplified by using the steam supply duct of the coffee machine (EP-A-0 509 505). The milk froth generating device comprises a laminarization cone solidly affixed to the steam supply duct under discharge openings at the otherwise closed end of the steam supply duct. The discharge openings are aligned, or inclined, so that the discharged steam jets are guided along the outer surface of the laminarization cone thereby dragging air for the frothing effect along, whereby a laminar flow of the steam is generated on the cone surface in order to improve the penetration of the steam jets into the milk and whereby in the milk into which the laminarization cone is partly submerged a turbulence is generated in order to improve frothing of the milk. This milk froth generating device has the above-described cleaning problem since the laminarization cone has to be submerged in the milk. The laminarization cone, furthermore, has to solve two contrasting partial tasks, namely dragging air along to perform the frothing effect and generating turbulence in the milk, on one hand, and to partly laminarize the steam jets for a deep penetration into the milk, on the other. For the preparation of hot beverages of instant powder, this prior art device has proved to be unsuitable.
The state of the art known from practice includes, furthermore, beverage preparing and vending machines where a vessel is provided into which, first, milk powder is dosed in and afterwards hot water essentially under atmospheric pressure is introduced whereupon, subsequently, the milk powder is dissolved by means of a machine-powered whisk, and by simultaneous intermixing ambient air milk froth is generated from it. Thereafter, the whisk can be taken from the vessel to be rinsed and espresso, or coffee, can be introduced below the froth to make the beverage ready. This milk froth generation is awkward particularly with a view to the required cleaning of the whisk, and the device to be used for performing the process is mechanically complicated.